When mounting a semiconductor chip on a substrate using a reflow soldering method, solder paste is used. When an amount of printed solder paste is excessive or when a force of pushing a semiconductor chip is excessively strong, the solder paste overflows from a soldered surface of the semiconductor chip and is subjected to reflow heating. This leads to formation of solder balls or uncontrollable spreading of molten solder paste to a flow area.
To deal with solder ball formation and uncontrollable spreading of molten solder paste to the flow area, conventionally, a buildup layer via-hole is provided so as to be connected to a pad on a printed circuit board, and a portion of copper plating inside the via-hole is removed to partially expose resin which is formed underneath the copper plating. This leads to forming of a via-hole inner wall portion where the resin is partially exposed. One such implementation is disclosed in Japanese Unexamined Patent Publication No. 2000-307224. In accordance with this configuration, the solder paste that overflows from the pad first, collects in the via-hole, and the molten solder in the via-hole is supplied to a necessary area at the time of reflow heating, and further the surplus solder gravitates toward the remaining conductor inside the via-hole.
However, since the conventional substrate structure is constructed so that the molten solder is drained to the inside of the via-hole which is a closed space, there is a fear that flux evaporates and increases pressure in the closed space, which causes explosion. As a result, there is also a possibility that solder balls scatter circumferentially and the semiconductor chip cannot be mounted in a desired state.
Furthermore, the conventional substrate structure is only for use in printed circuit boards, and cannot be used for a substrate where a semiconductor chip is mounted in a state in which the whole bottom surface of the semiconductor chip is in area contact with a copper plate formed in an upper surface of a substrate via solder paste. For this reason, there is no space to form a via-hole at all in the substrate.